Have you tried wet heat.... the heating pad has like a sponge layer in it and you wet that and put it in the sack with heating pad and use it..... feel better.

The Venture Infrared heatpad

Yes constantly need hot bottle

I get relief from heat on my back but unfortunately as soon as i stop the pain comes back worse

No unfortunately after a short period of time it doesnt help at all n the pain rerurns with a vengeance.